Systems for detecting object presence and location in a field by using a radiation beam which rotates around the field are disclosed in GB 2,196,114 (Hoare) and WO87/05688 (Fenner et al.). Both of these systems project radiation beams or light beams across the surface and receive the light at one or more opposed receivers. When an object is present in the field, it casts a shadow in an imaginary line between the light source and receiver. Both of these systems operate on the principle that if two such imaginary lines can be detected at different angular positions as the transmitter rotates around the field, then the position of the object may be detected from the intersection of the two lines. GB 2,196,114 (Hoare) discloses a dartboard implementation of the invention wherein a single light source and light detector rotate as a pair around the circumference of a dartboard surface. The scheme in this patent causes a dead area in the bullseye of the dartboard. No points in the dead area are detectable. Thus, darts that hit the bullseye are not detected. To detect objects in the dead area, it is necessary to physically move the scanning apparatus in a sideways direction. This additional step increases the complexity and scan time of the scanning process. WO87/05688 (Fenner et al.) also discloses an object detection device which may be used for locating darts on a dart board surface. The device uses one or more transmitters and receivers arranged around the edge of the surface. The preferred embodiment in Fenner et al. uses one or more movable transmitters which emit either narrow or divergent (broadcast) light beams that are received by stationary receivers. Alternatively, the transmitter(s) may be fixed and the receiver(s) may be movable. FIG. 1a of Fenner et al. shows a narrow light beam emitted from a single movable transmitter which is detected individually by a plurality of fixed receivers. FIG. 1b of Fenner et al. shows a divergent light beam emitted from a single movable transmitter which is simultaneously detected by a grouping of a plurality of stationary receivers.
When using automated devices to detect objects on a surface, such as darts on a dartboard, position accuracy is of primary importance. Dart players must be able to rely upon the results of the automated scorer under a variety of adverse conditions. Darts may be thrown in rapid succession, thereby requiring the automated scorer to work quickly. Leaner darts must be accurately placed in the right segment. A dart which hits very close to a previously thrown dart must be accurately detected. Darts must be accurately detected anywhere on the dart surface. It is also preferred that the automated scorer be inexpensive, easy to manufacture, and able to be retrofitted to conventional dartboards. None of the prior art schemes meet all of these criteria. The present invention fulfills the previously unmet need for an object presence and location detection device which meets all of these criteria.